U.S. Pat. No. 7,100,551 B2 discloses a carburetor that is incorporated in a stratified scavenging two-stroke engine. Two-stroke internal-combustion engines of stratified scavenging type are described in detail in Japanese Patent Laid-Open No. JP2002-227653 and International Publication No. WO 98/57053.
A stratified scavenging two-stroke engine introduces leading air into a combustion chamber at an early stage of a scavenging stroke, then introduces an air-fuel mixture into the combustion chamber to perform a scavenging process. A carburetor incorporated in the stratified scavenging type engine includes a fresh air passage supplying air filtered by an air cleaner to an engine body, and an air-fuel mixture passage creating an air-fuel mixture to supply the air-fuel mixture to the engine body (crankcase) when a throttle valve is in a fully-opened state (high speed operation state). The fresh air passage in the carburetor is connected via a piston groove or a reed valve to a scavenging passage communicating with the crankcase and the combustion chamber.
For a stratified scavenging engine, it is desirable that the whole amount of the air-fuel mixture created by the air-fuel mixture passage in the carburetor fills the crankcase of the engine body in a high-speed operation in which the throttle is fully opened. It is also desirable that fresh air that does not contain fuel component fills the upper portion of the above-described scavenging passage, the amount of fresh air being corresponding to the amount of blow-by of unburnt gas.
FIG. 20 illustrates a conventional and typical carburetor, focusing on a portion including a throttle valve (in a fully-opened state), in which FIG. 20(A) is a plan view and FIG. 20(B) is a longitudinal sectional view. In these figures, the reference numeral 900 denotes a carburetor, the reference numeral 902 denotes an intake air passage, the reference numeral 904 denotes a throttle valve, and the reference numeral 904a denotes a throttle valve shaft.
Referring to FIG. 20(B), a fresh air passage 906 is formed on one side of the throttle valve 904 and an air-fuel mixture passage 908 on the other side of the throttle valve 904 in the intake air passage 902, the side being defined by the throttle valve 904, when the throttle valve 904 is in its fully-opened state. On the side of the air-fuel mixture passage 908, there is provided a main nozzle or a main port 910, and an air-fuel mixture is created by the fuel discharged from the main nozzle or the main port 910.
In the figure, the arrow A denotes a flow direction of gas in the intake air passage 902. In addition, the reference numeral 912 denotes a well-known venturi section.
Referring to FIG. 20(A), when the throttle valve 904 in the fully-opened state is viewed in its plan view, while the throttle valve 904 is in the fully-opened state (high-speed operation), it is desirable to prevent mixing of the air-fuel mixture and the fresh air around the region indicated by the shaded portion of the throttle valve 904.
U.S. Pat. No. 7,100,551 B2 discloses two types of carburetors. The carburetor of one type is a carburetor that only incorporates a throttle valve and does not include a choke valve. The carburetor of the other type is a carburetor that incorporates a throttle valve and a choke valve. In these two types of the carburetors, an intake air passage in the carburetors is divided into a fresh air passage and an air-fuel mixture passage when the throttle valve is in a fully-opened state. The fresh air passage supplies air filtered by an air cleaner to the engine body (upper portion of the scavenging passage) via a piston groove. The air-fuel mixture passage supplies an air-fuel mixture created in this air-fuel mixture passage to the engine body (crankcase).
With reference made, as an exemplary one, to the carburetor illustrated in FIG. 3 of U.S. Pat. No. 7,100,551 B2, the carburetor includes a choke valve and a throttle valve. Inside of the carburetor, there is provided a first partition arranged on an upstream side of the choke valve. In addition, a second partition is provided on a downstream side of the throttle valve. Further, a third intermediate partition is provided between the first partition and the second partition. These first to third partitions reside on the same plane with respect to the choke valve and the throttle valve that are both in the fully-opened state.
The end faces of the first to third partitions, in other words, the end faces facing the choke valve or the throttle valve, have a circular arc shape. Specifically, the circular arc shape is a complementary shape with respect to the circular shape of the outer peripheral edge of the choke valve or the throttle valve.
According to the carburetor disclosed by U.S. Pat. No. 7,100,551 B2, it is made possible to physically partition the intake air passage of the carburetor into the above-described fresh air passage and the air-fuel mixture passage by reducing the clearance between each of the edges of the first to third partitions and the outer peripheral edges of the choke valve and the throttle valve, and by virtue of the choke valve and the throttle valve that are both in the fully-opened state, and thereby prevent mixing of the air and the air-fuel mixture.